Supervisor:

Submitted By:

Mr. Ashok Kumar Kajla

Rupesh Sharma

Associate Professr Enrolment No.:

10E2AIDCM1XT614

Design OFDM System and Overcome Nonlinear

Distortion in OFDM Signal at Transmitter Using

Peak To Average Power Ratio (PAPR) Reduction

Techniques

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Outline Project Description

Overview of OFDM

Single-Carrier vs. Multi-Carrier Transmission

Orthogonality

Cyclic Prefix (CP)

OFDM Modulation and Demodulation implemented using IDFT/DFT

Theoretical analysis OFDM system performance

Introduction of Peak-to-average power ratio(PAPR)

PAPR Reduction Techniques

Simulation Results For, OFDM system and PAPR reduction Techniques

Conclusions

References

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PROJECT DESCRIPTION

In this Dissertation we are discuses on ...• Theoretical analysis of OFDM system• Quantitative study of PAPR and

Nonlinear Distortion• Simulate an OFDM-system• Implement PAPR reduction techniques

Overview of OFDMOFDM is a powerful spectrally efficient digital

modulation technique. which employs multiple carriersthat are mutually orthogonal to each other over agiven time interval.

OFDM is also called a multicarrier modulationtechnology which is used in many new and emergingbroadband wireless communication systems like Wi-MAX, DVB-T and future 4G/LTE systems because of

1. Immunity to frequency selective fading channels

2. OFDM offer high spectral efficiency

3. Immune to the multipath delay

4. Low inter symbol interference (ISI)

5. High Power spectral EfficiencyDue to these merits OFDM is chosen as high data

rate communication system

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Drawbacks:1. The major drawback of OFDM is the high

PAPR of the transmitted signal. So system isSensitivity to nonlinear amplification.

Compensation techniques for nonlinear effects

1. Linearization (digital predistortion).2. Peak-to-average power ratio (PAPR)

reduction.3. Post-processing.

PAPR-reduction techniques1. Amplitude Clipping(AC) ,2. Selected Mapping Technique(SLM) and

3. Partial Transmit Sequence (PTS),5

Single-Carrier vs. Multi-Carrier

Transmission

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Single-Carrier commutation system1. For Single-Carrier commutation system consider

a bandwidth of B2. Transmit one symbols every T second T=1/B3. Single-Carrier commutation symbols rate = 1/T4. So in Single-Carrier commutation occupies the

entered commutation bandwidth B for one symbol

Single-Carrier commutation system model

multiple carriers can be

used for high rate data

transmission

multiple carriers is

overcome the frequency

selectivity of the wideband

channel

In the multichannel system

, let the wide band be

divided into N narrow band

sub channels , which have

the subcarrier frequency of

fk, k = 0, 1, 2,…, N-1.

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Multi-Carrier Transmission

Comparison of FDM and OFDM

FDM OFDM

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non-orthogonal nature

of carrier frequencies

in FDM

a large band gap is

required to avoid

inter-channel

interference, which

reduces the overall

spectral efficiency

subcarriers are

orthogonal to each

other in OFDM

These subcarriers are

orthogonal to each

other, thus required

no band gap which

improves the spectral

efficiency

It converts a

frequency selective

fading channel into

several fading

Comparison of FDM and

OFDM

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Basic Principles of

OFDM

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Standardized for IEEE 802.11a

The concept of OFDM is very much similar to

technique of FDM. FDM to allow multiple

messages to be sent over a single radio

channel.

If FDM is implement much more controlled

manner, then we can improved spectral

efficiency

OFDM is to split a high data rate streams(R=B)

into a number of lower data rate streams(R=B/N)

Due to this parallel transmission, the symbol

duration increases thus decreases the relative

amount of dispersion in time caused by multipath

Orthogonality

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Orthogonality is a property that allows multiple

information signals to be transmitted perfectly over a

common channel with the successful detection

Two functions or signals are said to be orthogonal if

they are mutually independent of each other

Orthogonality the two vectors are said to be

orthogonal if dot product is zero Sine and cosine

are best example of orthogonal signal and

integration of protect of the two orthogonal is

zero

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13

=

Frequency response of the sub-

carriers in a 5 tone OFDM signal

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OFDM transmission scheme implemented using

IDFT/DFT

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OFDM Based on the fast Fourier transform

In practice, discrete Fourier transform (DFT) and inverse

DFT (IDFT) processes are useful for implementing these

orthogonal signals.

Note that DFT and IDFT No. of operation (4(N* N)) can be

implemented efficiently by using fast Fourier transform (FFT)

and inverse fast Fourier transform (IFFT) no of operation

4Nlog2N, respectively.

In the OFDM transmission system, N-point IFFT is taken for

the transmitted symbols

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OFDM Modulation and Demodulation

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Let Xl{k} denote the lth transmit symbol at the kthsubcarrier, l =0, 1, 2….∞ ;1, k = 0, 1, 2,……N-1.Due to the S/P conversion, the duration oftransmission time for N symbols is extended toNTs, which forms a single OFDM symbol with alength of Tsym

(i.e., Tsym = NTs).

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Block Diagram of OFDM Modulation and

Demodulation:

Cyclic Prefix (CP)

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The OFDM guard interval

can be inserted in two

different ways.

One is the zero padding

(ZP) that pads the guard

interval with zeros.

The other is the cyclic

extension of the OFDM

symbol (for some continuity)

with CP (cyclic prefix) or CS

(cyclic suffix).

CP is to extend the OFDM

symbol by copying the last

samples of the OFDM

symbol into its front

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Effect of a multipath channel on OFDM symbols

with CP.

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Introduction of PAPR

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The transmit signals in an OFDM system can

have high peak values in the time domain since

In IFFT operation many subcarrier components

are added in same phase Therefore, OFDM

systems are known to have a high PAPR

compared with single-carrier systems

It decreases the efficiency of the power amplifier

in the transmitter

PAPR Definition: The PAPR of the discrete time baseband OFDM

signal is defined as the ratio of the maximum peak

power divided by the average power of the OFDM

signal.

With

where E{·} denotes the expected value.24

Distribution of PAPR in OFDM

System

Complementary Cumulative Distribution

Function(CCDF)

can be used to estimate

CCDF curves provide critical information about the

signals in 3G systems.

Evaluate the performance of any PAPR reduction

scheme.

Can be used to calculate BER.

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What are CCDF curves?

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A CCDF curve shows

how much time the signal

spends at or above a

given power level. The

power level is expressed

in dB relative to the

average power.

The percentage of time

the signal spends at or

above each line defines

the probability for that

particular power level.

A CCDF curve is a plot of

relative power levels

versus probability.

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The probability of PAPR is grater then or equal to 7.5 dB is

1 %t of the time. {FX(x) ccdf = P[PAPR X(7.5dB)>x 1%}

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function [] = ccdf_log_plot(x)

Th = 20*log10([0.1:0.1:10]);

kk = 1;

th_current = Th(1);

rms = sqrt(mean((x.^2)));

w = 20*log10(x/rms);

z = sort(w);

L = -inf*ones(1, length(Th));

for ii = 1:(length(z)-1)

if (z(ii) > th_current)

L(kk) = ii - 1;

kk = kk + 1;

th_current = Th(kk);

end

end

CCDF = (length(z) - L)/length(z);

semilogy(Th, CCDF);

grid on;

PAPR Reduction Techniques

PAPR reduction techniques are

classified into the different approaches

Clipping

Partial Transmission Sequence(PTS)

Selective Mapping(SLM)

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Amplitude Clipping

Clip the parts of OFDM signal at transmitter which are

outside the allowable limits.

Clipping method introduce both In band distortion and

out of band radiation into OFDM signals.

drawbacks:

BER performance degradation

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Selected Mapping (SLM)

Input sequences are multiplied by phase

sequences to generate alternative symbol

sequences.

Ability of PAPR reduction for selective mapping

depends upon number of required bits and their

design.

SLM can produce multiple time domain OFDM

signals that are asymptotically independent

Advantage: No distortion is introduced

× Drawback: It requires transmitting bits of side information per OFDM symbol

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Block diagram of selective mapping

(SLM) technique for PAPR reduction

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Partial Transmit Sequences

(PTS)

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The partial transmit sequence (PTS) technique

partitions an input data block of N symbols into V

disjoint sub blocks as follows:

where Xi are the sub blocks

SLM technique in which scrambling is applied to all

subcarriers,

scrambling (rotating its phase independently) is

applied to each sub block in the PTS technique

each partitioned sub block is multiplied by a

corresponding complex phase factor

V =1; 2; . . . ; V, subsequently taking its IFFT

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The original data block is partitioned into Vdisjoint sub blocks. The subcarriers in eachsub block are rotated by the same phasefactor such the PAPR of the combination isminimized.

At the receiver the reverse operation isperformed to recover the original data block.

Advantage: No distortion is introduced

× Drawback: It requires transmitting bits of sideinformation per OFDM symbol.

» It is crucial that the side information isreceived without errors.» The side information has to be heavilyprotected.36

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SIMULATION

RESULTS

Parameters use in OFDM system

No. of bits transmitted = 960000

Single frame size: 96 bits

Total no. of Frames: 10000

No. of Carriers: N=64

coding used: Convolution coding

Modulation: 16-QAM

Cyclic Extension: 25%(16)

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Generate

Random bits

Map(QAM-16 modulation )

Encoding

Data

IFFT

Adding cyclic

Extension (add

CP)

Channel

IFFT

Remove cyclic

Extension

(remove CP)

De Map(QAM-16

demodulation)

Decoding

Data

Rx. Random

bits

SNR(dB) BER(using 16-QAM)

0 0.4677

1 0.4467

2 0.4416

4 0.4086

6 0.3215

8 0.2520

10 0.1346

12 0.0290

14 0.0052

16 0

Table 1:

BER results for OFDM model

using 16- QAM modulation40

Figure 1:

SNR v/s BER plots for OFDM

system using subcarrier modulation

schemes 16 QAM

SNR v/s BER plots for OFDM

system

QAM-16 symbols before IFFT

Figure 2: Transmit QAM-16 symbols before IFFT (at the

Transmitter)41

42Figure 3: Receive QAM-16 symbols after FFT (at the

Receiver)

QAM-16 symbols after FFT

Figure 4: Transmit symbols after IFFT showing peaks without PAPR

suppression.43

SIGNAL BEHAVIOUR OF BASIC OFDM SYSTEM

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Figure 5: Transmit symbols after IFFT showing peaks real and Imaginary value of

signal without PAPR suppression.

Peaks of real and Imaginary value of signal

Amplitude Clipping Technique

Figure .6:. PAPR With and Without Reduction amplitude clipping with threshold 6 db and

with 10000 OFDM packets 45

Degradation of The Bit Error Rate (BER)

Performance of The System in Clipping Technique:

Figure 7: OFDM With and Without Clipping (BER V/S SNR)46

Selected Mapping Technique(SLM)

Figure 8: OFDM PAPR with and Without Reduction SLM with 10000 OFDM

packets47

Partial Transmit Sequence Technique(PTS)

Figure 9: OFDM PAPR With and Without Reduction PTS with 10000 OFDM

packets48

Comparison between PAPR Reduction Techniques

Reduction

TechniquesPAPR at CCDF of

Conventional

PAPR(dB)Proposed

PAPR(dB)

Difference of

PAPR Reduction in

(dB)

Clipping

Technique

(0.001%)10-3 8.94 6 2.94

(0.00001%)10-5 11.1 6 5.1

Selected

Mapping

Technique

10-3 9 8.1 0.9

10-5 11.1 10.4 0.7

Partial

Transmit

Sequence

Technique

10-3 8.93 7.5 1.43

10-5 12.6 10 2.649

Overall Analysis of Different Techniques:

Name of

schemes

Name of parameters

Signal

Distortion

Power

Increase

Data rate

Loss

Computational Complexity

Transmitter

processing

Receiver

processing

Amplitude

clippingYes No No

Amplitude

clippingnone

(PTS) No No Yes

M IFFTs ,WM 1

complex vector

sums

Side information

extraction,

inverse PTS

(SLM) No No Yes U IFFTs

Side information

extraction,

inverse SLM

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Applications of OFDM

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Some applications of OFDM in modern wireless digital

transmission systems are mentioned below:

Asynchronous Digital Subscriber Line (ADSL), High

speed DSL, Very high speed DSL use OFDM for

transmission of high rate data transfer

Digital Audio Broadcasting (DAB) and Digital Video

Broadcasting (DVB).

IEEE 802.11a, IEEE 802.11g and HYPERLAN2

wireless Local Area Network (WLAN) standards include

OFDM for supporting higher bit rates

IEEE 802.16 Wireless Metropolitan Area Network

(MAN) standard also includes OFDM.

Conclusion Three PAPR reduction techniques are discussed as

clipping , SLM, and PTS .

clipping is the simplest method to reduce the PAPR.

however , low efficiency and high BER is drawback

of clipping .

Both SLM and PTS are important probabilistic

schemes for PAPR reduction .

PTS performs better than SLM in terms of PAPR

reduction provided PTS is less computational

complexity than SLM(since PTS can avoid several

IFFT operations)

But no of required bits as side information is larger

in PTS than SLM.52

References1. Y.Wu and W. Y. Zou, “Orthogonal frequency division multiplexing: A multi-carrier

modulation scheme,” IEEE Trans. Consumer Electronics, vol. 41, no. 3, pp.

392–399, Aug. 1995.

2. R. van Nee and A. deWild, “Reducing the peak to average power ratio of

OFDM,” in Proceedings of the 48th IEEE Semiannual Vehicular Technology

Conference, May 1998, vol. 3, pp. 2072–2076.

3. PAPR Reduction of OFDM Signals Using a Reduced Complexity PTS

Technique Seung Hee Han, Student Member, IEEE, and Jae Hong Lee, Senior

Member, IEEE

4. John G. Proakis and Masoud Salehi,”Digital Communications”, 5th Edition,

McGraw-Hill Higher Education 2008.

5. Tao Jiang and Yiyan Wu, “An Overview: Peak-to-Average power Ratio

Reduction Techniques for OFDM Signals”, IEEE Transactions on Broadcasting,

Vol. 54, NO.2, June 2008.

6. Taewon et al, “OFDM and its Wireless Applications: A survey”, IEEE

Transaction on Vehicular Technology, Vol. 58, NO.4, May 2009.

7. Dae-Woon Lim et al, “An Overview of Peak-to-Average Power Ratio Reduction

Schemes for OFDM Signals”, Journal of Communications and Networks, Vol.

11, NO.3, June 2009.

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Questions & Answers

Thanks

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